Echilibrul biologic depinde de bacteriile benefice.

Because -ul is the Romanian definite article attached to the end of the noun.

• echilibru = balance
• echilibrul = the balance

Romanian often uses the definite form in abstract, scientific, or general statements where English may or may not use the.

Because echilibru is a neuter singular noun.

In Romanian, neuter nouns behave like masculine in the singular and like feminine in the plural. So in the singular, the adjective takes the masculine singular form:

• echilibru biologic = biological balance

If the noun were plural, the adjective would change too:

• echilibre biologice = biological balances

In Romanian, the normal, neutral position for most adjectives is after the noun.

So:

• echilibrul biologic
• bacteriile benefice

Putting the adjective before the noun is possible in some cases, but it usually sounds more literary, emphatic, or stylistically marked. In a neutral scientific sentence, noun + adjective is the expected order.

Depinde is the 3rd person singular present tense of the verb a depinde, meaning to depend.

It is singular because the subject is singular:

• Echilibrul biologic depinde... = Biological balance depends...

Compare:

• depind = they depend
• depinde = it depends

After a depinde, Romanian uses the preposition de as part of a fixed pattern:

• a depinde de = to depend on

So this is something best learned as a whole expression, not word by word.

Examples:

• Depinde de vreme. = It depends on the weather.
• Succesul depinde de muncă. = Success depends on work.

It is in the accusative, because the preposition de normally takes the accusative.

In this sentence, you do not see a special visible change, because for many Romanian nouns the nominative and accusative look the same. So the form stays bacteriile benefice.

Because it is built in steps:

• bacterie = bacterium / bacterium-type singular noun
• bacterii = bacteria
• bacteriile = the bacteria

So bacteriile is basically the plural form bacterii plus the definite article -le.

This kind of vowel sequence is normal in Romanian, especially with nouns ending in -ie.

Because bacteriile is the definite plural form, while bacterii is the indefinite plural form.

• bacterii benefice = beneficial bacteria
• bacteriile benefice = the beneficial bacteria

In Romanian, the definite form is often used more broadly than in English, especially in general or scientific statements. So bacteriile benefice can refer to beneficial bacteria as a class, or to the beneficial bacteria relevant in the context.

Because the adjective must agree with bacteriile.

The noun bacterie is feminine, and here it is plural, so the adjective must also be feminine plural:

• masculine singular: benefic
• feminine singular: benefică
• masculine plural: benefici
• feminine plural: benefice

That is why the sentence has bacteriile benefice.

Yes, Romanian word order is fairly flexible, but the sentence as given is the most neutral and standard order:

• Echilibrul biologic depinde de bacteriile benefice.

Other orders are possible for emphasis, contrast, or style, but they are less neutral. For example, moving de bacteriile benefice to the front would put stronger focus on that phrase.

So for a normal declarative sentence, the given order is the best one to learn first.